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  3. "The Evolution of Birds" The Origin of Birds Analysis of birds and of reptilian fossils indicate that birds belong to the

"The Evolution of Birds" The Origin of Birds Analysis of birds and of reptilian fossils indicate that birds belong to the

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问题 "The Evolution of Birds"
   The Origin of Birds
   Analysis of birds and of reptilian fossils indicate that birds belong to the group called therapods. Several species of dinosaurs closely related to birds had feathers with vanes, and a wider range of species had filamentous feathers. Such findings imply that feathers evolved long before powered flight.
   Among the possible functions of these early feathers were insulation, camouflage, and courtship display.
   Derived Characteristics of Birds
   Many of the characteristics of birds are adaptations that facilitate flight, including weight-saving modifications that make flying more efficient. For example, birds lack a urinary bladder, and the females of most species have only one ovary. Living birds are also toothless, an adaptation that trims the weight of the head.
   A bird’s most obvious adaptations for flight are its wings and feathers. Feathers are made of the protein B-keratin, which is also found in the scales of other reptiles. The shape and arrangement of the feathers form the wings into airfoils, and they illustrate some of the same principles of aerodynamics as the wings of an airplane. Power for flapping the wings comes from contractions of large pectoral (breast) muscles anchored to a keel on the sternum (breastbone). Some birds, such as eagles and hawks, have wings adapted for soaring on air currents and flap their wings only occasionally; other birds, including hummingbirds, must flap their wings continuously to stay aloft. Among the fastest birds are the appropriately named swifts, which can fly up to 170 km/hr.
   A    Flight provides numerous benefits. B    It enhances hunting and scavenging; many birds consume flying insects, an abundant, highly nutritious food resource. C    Flight also provides ready escape from earthbound predators and enables some birds to migrate great distances to exploit different food resources and seasonal breeding areas. D
   Flying requires a great expenditure of energy from an active metabolism. Birds are endothermic; they use their own metabolic heat to maintain a high, constant body temperature. Feathers, and in some species layers of fat, provide insulation that enables birds to retain their body heat. The lungs have tiny tubes leading to and from elastic air sacs that improve airflow and oxygen uptake. This efficient respiratory system with a four-chambered heart keep tissues well supplied with oxygen and nutrients, supporting a high rate of metabolism.
   Flight also requires both acute vision and fine muscle control. Birds have excellent eyesight. The visual and motor areas of the brain are well developed, and the brain is proportionately larger than those of amphibians and nonbird reptiles. Birds generally display very complex behaviors, particularly during breeding season, when they engage in elaborate courtship rituals.
   How did flight evolve in the therapods? In one scenario, feathers may have enabled the small, running dinosaurs chasing prey or escaping predators to gain extra lift as they jumped up into the air. Or, small dinosaurs could have gained traction as they ran up hills by flapping their feathered forelimbs—a behavior seen in birds today. In a third scenario, some dinosaurs could have climbed trees and glided, aided by feathers. Whether birds took to the air from the ground up or from the trees down, an essential question being studied by scientists ranging from paleontologists to engineers is how their efficient flight stroke evolved.
   By 150 million years ago, feathered therapods had evolved into birds. Archaeopteryx, which was discovered in a German limestone quarry in 1861, remains the earliest known bird. It had feathered wings but retained ancestral characteristics such as teeth, clawed digits in its wings, and a long tail. Archaeopteryx flew well at high speeds, but unlike a present-day bird, it could not take off from a standing position. Fossils of later birds from the Cretaceous show a gradual loss of certain ancestral dinosaur features, such as teeth and clawed forelimbs, as well as the acquisition of innovations found in extant birds, including a short tail covered by a fan of feathers.
   Glossary
   Cretaceous: a time period, 144-65 million years ago
The word their in the passage refers to
选项 A、feathers
B、species
C、layers
D、birds
答案D
解析 "Feathers, and in some species, layers of fat provide insulation that enables birds to retain their [birds] body heat." The pronoun "their" does not refer to Choices A, B, or C.
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